CD103+ dendritic cells (CD103+ DCs) are a critical immune subset that resides in nonlymphoid organs in the mouse such as skin, liver and lung. This DC population migrates to lymph nodes to induce adaptive immune responses and is notably proficient in cross-presentation of exogenous antigen to cytotoxic CD8+ T cells. Because of their tissue residence and function, CD103+ DCs are perfectly situated to regulate immune responses to tumors. However, little is understood about their role in cancer. CD103+ DCs are homologous to human tissue CD141+ DCs and DCs generated in GM-CSF for cancer therapy, and provide a genetically tractable system to understand these important DC populations. Recently, our lab and others found CD103+ DCs have a unique developmental pathway dependent on the cytokine GM-CSF, the signal transducer and activator of transcription STAT5 and the transcriptional regulator Id2. This signaling pathway correlates with enhanced DC-mediated tumor immunity. By contrast, we found that the tumor-associated cytokine IL-6 represses Id2 expression in DCs via STAT3 signaling, rendering reduced DC-mediated tumor immunity. The cellular and molecular mechanisms by which STAT5 and STAT3 control CD103+ DC functional responses remain unclear. These gaps in knowledge are important to address to provide better use of DCs in immunotherapy. Based on our data, we hypothesize that GM-CSF-STAT5 and IL-6-STAT3 signaling have opposite roles in regulating DC anti-tumor activity via their control of Id2 expression in DCs. We will test this hypothesis wit 3 project aims. In Aim 1, we will delineate mechanisms by which tissue- resident DCs stimulate anti-tumor immunity by investigating the roles for STAT5 and its target gene Id2 in CD103+ DCs. We will study whether and how STAT5 and Id2 control CD103+ DC maturation, antigen presentation, T lymphocyte stimulation, pre-DC accrual in nonlymphoid tissue, and DC-mediated anti-tumor activity. In Aim 2, we will elucidate DC-intrinsic signaling mechanisms that regulate DC function in melanoma tumors. We will investigate roles for STAT3 in DCs in Id2 downregulation, immune responses to melanoma and melanoma combination immunotherapy. In Aim 3, we will examine mechanisms by which STAT5 and STAT3 regulate Id2 expression in DC development. We will evaluate whether and how STAT5 and STAT3 control chromatin modifications, co-factor recruitment and RNA polymerase II association at the Id2 promoter in vivo. We anticipate this project to reveal cellular and molecular mechanisms by which CD103+ DC anti-tumor activity is regulated, paving the way for rational investigation into pathways controlling related human DC populations. This information could ultimately provide new opportunities to manipulate DCs in cellular therapy, cancer and/or immune disease.